In certain circumstances it may be desirable to combine image information from two or more sources to provide a composite image. One such occasion is where it is desirable to provide text data (such as range or similar) over an image. Another such occasion is where it may be desirable to combine image data from an image intensifier (I2) with image data obtained from a thermal imager. An image intensifier amplifies visible light and near infra red light (say 400 nm-900 nm in wavelength); whilst a thermal imager operates in a further portion of the infra red light spectrum (say 7000 nm-14000 nm in wavelength). This is advantageous as the longer wavelengths are generally less attenuated by smoke and dust, although image intensifiers generally have better resolution than thermal imagers.
Current night vision systems using image intensification benefit from higher resolution and more natural imaging in comparison to imaging in the far infrared. However, I2 systems are limited in so far as they do need at least some ambient light to operate. A typical intensifier tube becomes noise limited at around 100 μlux which corresponds to a moonless overcast night sky. At this point and at lower light levels effective night vision requires infrared systems.
Thermal imaging also offers advantages over I2 in detecting targets concealed by camouflage. Addressing I2 limitations by including a thermal imaging capability improves overall night vision performance but at the expense of complexity and weight. Hand held systems are easy to interchange when each system is self contained with its own optics, display and power supply. On the other hand helmet or system mounted image intensifiers cannot be easily swapped over.
An example of an apparatus that provides this image combination is disclosed in US2008/0302966. In this system, a conventional image intensifier is provided with a clip on attachment comprising a thermal imager, a projector operable to project a visible image captured by the thermal imager and a light turning element which turns light from the projector into the aperture of the image intensifier. In this manner light from the thermal imager and the image intensifier may be combined to provide a single image.
One potential disadvantage of this arrangement is that a small amount of the light emitted by the projector may be reflected off the front lens of the image intensifier or from the internal elements of the image intensifier. This reflected light may draw attention to a user of the device. In particular, this light may be more readily detected by others using image intensifier based devices. Whilst this issue can be partially mitigated by reducing the light output by the projector, this may not be practical or desirable in some circumstances.
It is therefore an object of the present invention to provide a reflection barrier for an optical data insertion device and to an optical data insertion device incorporating such a barrier that at least partly overcomes or alleviates the above problems.